Hydroxyapatite Ca.sub.10 (PO).sub.4).sub.6 (OH).sub.2 ! is a major mineral component in animal and human bodies, especially in "load-bearing" tissues such as bone and teeth. For example, in a typical wet cortical bone, which is composed of 22 wt % organic matrix, 69 wt % mineral, and 9 wt % water, the major subphase of the mineral component consists of submicroscopic crystals of an apatite of calcium and phosphate, whose crystal structure resembles that of hydroxyapatite (HA). (Triffit, Chapter 3, FUNDAMENTAL AND CLINICAL BONE PHYSIOLOGY, ed. M. R. Urist (1980)). The apatite crystals are usually formed as slender needles, 20-40 nm in length and 1.5-7 nm in diameter. (Hohling et al. (1974) Cell Tissue Res. 148:11). The mineral phase is not a discrete aggregation of calcium phosphate mineral crystals per se. Rather, it is made of a continuous cellular structure which provides good mechanical strength. The apatite family of minerals, A.sub.10 (BO.sub.4).sub.6 X.sub.2, crystallizes into a hexagonal rhombic prism. Hydroxyapatite, in particular, has the unit cell dimensions of a=0.9432-0.9418 nm and c=0.6881-0.6884 nm, and the maximum X-ray diffraction plane is (211). Posner et al. (1958) Acta Crystallogr. 11:308. The ideal Ca:P ratio of HA is 10:6 and the calculated density is 3.219 g cm.sup.-3. (See, McConnell (1973) APATITE: ITS CRYSTAL CHEMISTRY, Springer-Verlag, Berlin).
Calcium phosphate-based biomaterials have been in use in medicine and dentistry for over 20 years because of their excellent biocompatibility with human tissues. Thus, hydroxyapatite has been widely used in dental implants, percutaneous devices, periodontal treatment, alveolar ridge augmentation, orthopedics, maxillofacial surgery, otolaryngology, and spinal surgery. (Hench (1991) J. Am. Cer. Soc. 74:1487).
In addition, hydroxyapatite has also been used as a biological chromatography support in protein purification and DNA isolation. (Tiselius et al. (1956) Arch. Biochem. Biophys. 65:132; U.S. Pat. No. 4,798,886). Hydroxyapatite is also currently used for fractionation and purification of a wide variety of biological molecules, such as subclasses of enzymes, antibody fragments, and nucleic acids. (See, e.g., Kadoya et al. (1988) Liquid Chrom. 11(14):2951; Cummings et al. (1995) Bio Rad Bulletin 1927 US/EG REVA; Kurtz et al. (1977) PNAS 74:4791). Crystalline hydroxyapatite columns are commonly used in high-performance liquid chromatography. Typically, the chromatographic column is filled with irregularly shaped hydroxyapatite gel having poor mechanical strength.
It is known that spherical powders, in general, have better rheological properties than irregular powders and, thus, produce better coatings for hip implants and chromatographic separation. (Liu et al. (1994) J. Mater. Sci. Med. 5:147). Spherical hydroxyapatite ceramic beads have recently been developed that exhibit improved mechanical properties and physical and chemical stability. However, these spherical ceramic beads are between 20-80 .mu.m in size. (Cummings et al. (1995) Bio Rad Bulletin 1927 US/EG REV A.)
There are many advantages to reducing the granule size of spherical hydroxyapatite material. In general, the smaller the granule size, the higher the specific surface area and the higher the bonding capacity. U.S. Pat. No. 4,874,511 describes a column system for use in chromatography using a combination of hydroxyapatite particles having a diameter less than 1 .mu.m and aggregated fine particles having diameters between 1 and 10 .mu.m. Theoretically, the specific surface area (i.e., surface area per volume) is proportional to 6/d, where d is the diameter of the spherical granule. This relationship indicates that the specific surface area varies linearly with the reciprocal of the diameter of the granule. In addition, the mechanical properties of a packed column can be improved by reducing the granule size, resulting in more contacting surface area and greater frictional forces between granules. Furthermore, a uniform pack is expected to have a homogeneous pore distribution. However, very small crystals have been known to cause flow problems when used in chromatography.
Many of the published synthesis methods for HA are essentially chemical precipitation methods where controlled powder morphology and the nanocrystalline structure was not the primary concern. (Tanahashi et al. (1992) J. Mater. Sci. Med. 3:48; Li et al. (1993) J. Ater. Sci. Med. 4:127; Ybao et al. (1994) J. Mater. Sci. Med. 5:252). U.S. Pat. No. 4,335,086 to Spencer describes preparation of hydroxyapatite by heating an aqueous suspension of burshite to prepare rosette-shaped crystals. These crystals are between 40 and 70 .mu.m in size.
U.S. Pat. No. 4,371,484 to Inukai et al. describes a process for making porous, sintered calcium phosphate particles. A foaming agent is added to a calcium phosphate slurry which is then dipped into a porous body of organic material and heated to form a porous network of hydroxyapatite.
The use of spray-dryers or atomizers to produce fine particles is known in the art. U.S. Pat. No. 5,435,822 to Blouin describes a method for applying plant fertilizer using spray-dried nutrient components. U.S. Pat. Nos. 5,108,956 and 5,205,928 to Inoue et al. describe processes for preparing sintered microspherical hydroxyapatite particles by spray-firing a suspension of hydroxyapatite dispersed in an inflammable solvent into a flame. These particles can also be constructed to have specific surface area, porosity and mechanical strength characteristics.
U.S. Pat. No. 5,082,566 to Tagaya et al. describes a calcium-phosphate type hydroxyapatite from 0.5 to 50 .mu.m in diameter. The hydroxyapatite is formed by granulating an aqueous calcium-phosphate solution which is in the form of a gel or slurry by spray-drying the gel into a high-temperature air stream ranging from 100.degree. C. to 200.degree. C., thereby drying it instantaneously, and then subsequently firing the granular apatite at 400.degree. C. to 700.degree. C.
U.S. Pat. Nos. 5,039,408 and 5,441,635 to Ichitsuka et al. describe packing materials for chromatography columns. U.S. Pat. No. 5,039,408 describes fluoroapatite particles formed by firing fluoroapatite at between 900.degree. C. and 1400.degree. C. to obtain particles with pores of 0.01 to 20 .mu.m. In U.S. Pat. No. 5,441,635, Ichitsuka et al. describe hydroxyapatite particles which are coated with a surface layer of fluoroapatite. The hydroxyapatite particles are between 2 and 100 .mu.m and are formed by spray-drying a slurry of hydroxyapatite.
U.S. Pat. No. 5,158,756 to Ogawa et al. describes a method of producing spherical particles of calcium phosphate containing open pores which varied from 100 to 4000 .ANG.. Over 90% of pores have a pore size which is 0.5 to 2 times larger than the average pore size. In this method, a hydroxyapatite slurry is stirred until it reaches a viscosity of 20 cP and is then spray-dried at a temperature ranging from 700.degree. C. to 900.degree. C.
None of the known methods that utilize spray-drying produce hydroxyapatite particles having controlled morphologies and/or having homogenous cellular structures.
The present invention provides novel methods of producing controlled morphology spherical hydroxyapatite granules ranging in size from 1 to 8 .mu.m. Methods within the present invention involve the initial preparation of a hydroxyapatite slurry containing ammonium hydroxide followed by a spray-drying process, the controlling step, to produce granules having various structures. By adjusting the operating parameters (e.g., atomization pressure) and starting slurry (e.g., concentration), the methods of the subject invention can produce hollow or solid spheres and doughnut-shaped hydroxyapatite granules. The invention also provides methods of producing controlled morphology hydroxyapatite granules having homogenous cellular structures.